Yo, you're kind of on the correct path. I think though it probably wont work for you if you dont have a sound amplitute tool already (I know, counter intuitive).

Step1: for this to work, you need to be in a room where you have a device producing a sound of a certain loudness, use a sound loudness measuring tool and then put that value in the calibration field. By pressing calibrate it will calculate the offset, how far off the current reading of the app is from the actual value, and then it will try to take this into account when measuring sounds (and display the proper values).

Step2: Here you'd put the offset if you already knew it (if you had done step1 in the past for example). Since you haven't, you'd have to do step1 instead of step2.

Step3: yes

Step4: yes

Step5: I'd do both probably. Export data either as csv (comma separated values file) or as excel file and I'd also export a screenshot of the graph.

To conclude, you need a sound measuring tool, a constant sound source (like a phone playing a specific frequency at a certain volume) and a somewhat quiet place to avoid any interference. If you know anyone who could lend you their sound measuring tool to do a calibration it could help. Otherwise, there are cheap sound meters (maybe some Uni-T ones?).

PS. Even if the readings are correct though, I dont know if they could be considered as evidence, you'd might have to check that first.